In this editorial, I would like to highlight device-to-device (D2D) communication, which has emerged as a hot research area for next generation cellular networks. D2D communication typically aims to leverage the physical proximity of communicating devices to extend cellular coverage mostly in sparse deployment scenarios. D2D communication may have additional advantages such as offloading in cellular networks, improved spectrum reuse and system throughput, and so forth. However, it does pose new problems and challenges, some of which are mentioned as follows.